In recent years, the number of patients suffering from chronic respiratory diseases such as pulmonary emphysema, pulmonary tuberculosis sequelae, chronic bronchitis and the like has tended to increase because of progress in aging and deterioration in living conditions. Oxygen inhalation therapy to inhale highly concentrated oxygen has been used as an effective treatment method for such patients. Oxygen inhalation therapy is a treatment method for patients with diseases described above to inhale an oxygen gas or oxygen-concentrated gas. A supply source of oxygen gas or oxygen-concentrated gas used in treatment includes a high pressure oxygen cylinder, a liquefied oxygen cylinder, an oxygen concentrator and the like, but the case using the oxygen concentrator has been increased because of durability in continuous use for a long period of time, easiness in use and the like.
The oxygen concentrator is an apparatus capable of separating oxygen from air to concentrate. As such an apparatus to separate and concentrate oxygen, an adsorption-type oxygen concentrator, equipped with a single or plural adsorption beds filled with adsorbents capable of selectively adsorbing nitrogen in air is widely known and used in hospitals and at homes from a standpoint of generating highly concentrated oxygen of 90% or higher. Among them, a pressure swing adsorption-type oxygen concentrator using a compressor as a pressure swing apparatus is widely used. Such an apparatus is an apparatus to generate highly concentrated oxygen continuously by repeating, at a certain cycle, an adsorption step to obtain unadsorbed and concentrated oxygen by supplying compressed air from a compressor to single or plural adsorption beds filled with adsorbents capable of selectively adsorbing nitrogen and adsorbing nitrogen on the adsorbents under the conditions in which inside of the adsorption beds is pressurized, and a desorption step to desorb nitrogen from the adsorbents by reducing the inner pressure of the adsorption beds to an atmospheric pressure or to a vacuum level for regeneration.
Measures to fine-tune a setting range of a volume of air to be supplied from a compressor to the adsorption beds or correct an adsorption-desorption sequence to tolerate a change of surrounding conditions, particularly temperature dependency of the adsorbents to maintain the oxygen concentration are adopted to respond to a decrease of an oxygen concentration in an oxygen-concentrated gas generated, which is caused by a change of environmental conditions such as variation in used temperature or barometric pressure in the oxygen concentrator or a decrease of the oxygen concentration in an oxygen-concentrated gas generated, which is caused by deterioration of the equipment with time.
Such an apparatus to maintain a product oxygen concentration is known, wherein a change of the oxygen concentration with time is corrected or a decrease of the oxygen concentration caused by deterioration of the equipment with time is compensated and the oxygen concentration of an oxygen-concentrated gas is maintained at a constant value by detecting the oxygen concentration of the oxygen-concentrated gas by using an oxygen sensor and performing feedback controls on an airflow volume of a compressor, a cycle timing of adsorption-desorption and the like (Japan Patent Application Laid-Open No. 2000-516854 and Japanese Patent Laid-open publication No. Laid-Open H09-183601).